July 1, 2025

Is there a connection between bacteria in the mouth and gut and the development of dementia in Parkinson’s disease? New research may shed light.

Cognitive decline and dementia are part of Parkinson’s disease for most people. When a person develops dementia in the context of existing Parkinson’s disease, that is called Parkinson’s disease dementia (PDD), one of the two major forms of Lewy body dementia (LBD), the other being dementia with Lewy bodies (DLB). But what biological factors contribute to the appearance of dementia in a person with Parkinson’s disease? There are likely many, and some recent research suggests that the bacteria and other microorganisms that live in the mouth and gut—collectively called the oral and gut microbiome—may play an important role.

What was this study about?

While not yet well understood, the microbiome has drawn the attention of the research community due to the extraordinarily high number of microorganisms that live in and on our bodies, and the recognition that they affect our health profoundly – in both positive and negative ways. Moreover, the functioning of the gut is compromised in Parkinson’s disease, suggesting that the microbiome may have a special role in Parkinson’s. Given that other studies have linked the appearance and progress of dementia with changes in the microbiome, the researchers wanted to understand how the microbiome in the mouth and gut may differ between people without Parkinson’s (called “healthy controls”), people with Parkinson’s and mild cognitive impairment (PD-MCI) and people with PDD.

What did the researchers do?

The researchers worked with over 100 research volunteers – 26 healthy controls, 41 people with PD-MCI, and 47 people with PDD. Volunteers gave samples of their saliva and stool. The research team then analyzed the saliva and stool samples to learn about the bacteria present in the volunteers’ mouths and guts, respectively.

The researchers studied the DNA found in these samples to identify which bacteria were present in the saliva and stool samples and analyzed the results with both traditional techniques and AI.

What were the findings?

The key finding was that bacteria usually found in the mouth are commonly found in the guts of people with PD-MCI and PDD. This suggests that the movement of oral bacteria to the gut becomes more common as people with Parkinson’s disease develop cognitive decline and dementia. It is not yet clear if these changes are a result of the development of cognitive impairment, a contributor to the development of cognitive impairment, or if both bacteria and cognitive impairment are driven by another, as yet unknown, factor. However, the findings are consistent with a potential causative role of the movement of bacteria from the mouth to the gut.

Furthermore, the researchers found that some bacteria were more common in PD-MCI and others were more common only in PDD. This suggests that the relationship between the presence of these bacteria in the gut and cognitive decline is complex and changes over time. It also hints that someday doctors may be able to use bacterial measures to identify people with Parkinson’s who are at high risk for developing dementia. Moreover, these data may point toward potential therapeutic strategies aimed at controlling bacteria in the mouth and gut.

Will this change prevention strategies or medical treatment?

Not yet, but possibly in the future. This study adds to a growing body of research into the link between the microbiome and brain diseases such as LBD. However, this connection is still poorly understood, and much more research needs to be done before scientists and doctors can be confident in the strength and interpretation of the data. It is possible that modifying the microbiome could be a path toward preventing or treating dementia in Parkinson’s disease, but clinical trials would need to be designed and carried out to know that any therapeutic approach suggested by these results would be safe and effective.

How can I get involved?

Progress against LBD depends on research, and research depends on volunteer participants. If you or someone you know is would like to learn more about research studies that are currently seeking volunteers, you can visit the LBDA clinical studies page.

If you would like to be notified by email when new studies become available, please consider signing up for the Lewy Trial Tracker.

Reference:

Clasen F., Yildirim S, Arikan M, et al. 2025. Microbiome signatures of virulence in the oral-gut-brain axis influence Parkinson’s disease and cognitive decline pathophysiology. Gut Microbes, 17. DOI: https://doi.org/10.1080/19490976.2025.2506843